speakers with a "smooth" impedance curve


I had started a thread asking about speakers that are well designed for tube amps (I am currently using a Ming Da MC 34AB with 8 EL34 power tubes 75 wt/ch ultralinear; 40 wt/ch Class A).
There has been a consistent recommendation for efficient speakers with a "smooth impedance curve".

Any recommendations out there for some tower speakers in the less than $5000 price range with smooth impedance curves that are "tube friendly"?
rsasso
Well, I guess we cannot agree all the time, Duke...

Anyway, I probably understand where you are coming from, even if I disagree as far as the benefits go. I just would like to clarify a bit more so that I can at least say that we are more or less on the same page of what is at work here, even if our opinions diverge.

Along those lines, a few questions, if I may:
1) From your statement, I infer most of the direct experience you have with the network you mentioned implementing is in partnership with OTL amplification.

Is this correct?

2) The solid state amplification experience is indirect, through the customer.

Is this correct?

3) Normally, large impedance swings are "smoothed" by networks that flatten their peaks (though I know of one that works the other way), which, if we are interested in partnering with OTL amplification would kind of run counter to that. Though, without making an already long post even longer, I can understand the argument against the statement I just made, and in some instances, I even support it.

Can you provide a high-level description of this network you are referring to, please? Of course, I respect the fact that some folks are more sensitive to discussing things than others such as myself normally are. But, if it's a well-known, previously illuminated topology, as opposed to something truly unique, novel, and patentable that you would feel uncomfortable in discussing, please describe it.

Finally, again, I understand what most designers are aiming for with these impedance compensation networks. In fact, I accept that I'm likely representing the minority opinion here, as without naming names, what I consider to be the flagship loudspeaker of the current high-end audio marketplace implements them, as well as a highly successful company that offers tweaks as an add on to one's existing pair of loudspeakers. The benefits are more or less exactly as outlined in the words of the customer you quoted. Nevertheless, I disagree with their implementation in most instances, as I feel the detriments they add in terms of robbing the music of immediacy, openness, and dynamism seem incredibly difficult to overcome elsewhere in the audio chain.
as usual I agree with Duke. I'd just add that when a crossover is designed for a speaker to have a smooth impedance curve this can be done without using any filters or extra components, it just takes a lot more time etc., some companies do this some don't. Mine are built and tested using solid state amps, but smooth is smooth and the real difference between the SS and tube amps is what impedance range they like to see, both work best with a smooth load.
Hi Trelja,

Yeah we finally disagree. It's about time, don't you think??

The circuit I used is nothing you won't find in the Loudspeaker Design Cookbook, and it flattens an impedance peak somewhere around 2 kHz (the specifics are foggy now). I've used that and other impedance-smoothing techniques in speakers that have been hooked up to a wide variety of amps, but only used that one as an external add-on that one time... that I can remember. That case was interesting because it gave a before and after comparison. If you shoot me an e-mail I'll give you the guy's e-mail and you can contact him directly. There's a small possibility that you know him.

I've heard his speakers with and without the external impedance filter on his solid state amps, but his assessment is better qualified than mine and besides a designer patting himself on the back isn't credible, even if it's me! I prefer to let other people pat me on the back... or kick me in the butt, as the case may be.

Now some impedance-smoothing techniques are detrimental to clarity, as are some equalization techniques, in my opinion.

If you shoot me an e-mail I'll tell you what the basic circuit is (I don't have a record of the values that I used), but I'd rather not post it here. You've probably already figured it out. If you have a speaker in mind that you'd like to try it on, if you can get me a look at the impedance curve I'll see if it's feasible and if so make some suggestions as to values you might try. A candidate for this type of circuit would be a speaker that was designed for a solid state amp, which has a single peak in the impedance curve somewhere above the bass region, and you want to drive this speaker with a high output impedance tube amp like an SET or OTL.

Duke
If I can clarify something here-

If the speaker was voiced to work with transistors, the smoother impedance curve is going to be a real boon if you are trying to use tubes.

If the speaker is designed for tubes, the impedance curve likely will not matter too much. So both Duke and Trelja are correct as this is an issue that is very use-specific.

for more information see:
http://www.atma-sphere.com/papers/paradigm_paper2.html
Daedalus, what you describe is what I do in my speakers, instead of using an outboard impedance equalization filter. And I would estimate that designing for a smooth impedance curve as well as a smooth frequency response curve roughly quadruples the workload it takes to get the crossover right. But then as you said smooth is smooth, and amps like that.

And by the way, your crossovers are particularly well done; I can tell that attention was paid to the power response, not just the on-axis response.